Joe Biden warned that America’s enemies would test Barack Obama with an international crisis within six months if he’s elected president – a shocking comment John McCain eagerly pounced on yesterday to claim Obama isn’t ready to be commander-in-chief.

“It will not be six months before the world tests Barack Obama like they did John Kennedy. The world is looking. We’re about to elect a brilliant 47-year-old senator president of the United States of America.

“Watch. We’re going to have an international crisis, a generated crisis, to test the mettle of this guy.

“And he’s going to need help . . . to stand with him. Because it’s not going to be apparent initially; it’s not going to be apparent that we’re right.”

McCain treated Biden’s comments as a gift while stumping across Missouri yesterday.

“The next president won’t have time to get used to the office. We face many challenges here at home and many enemies abroad in this dangerous world,” McCain said. “We don’t want a president who invites ‘testing’ from the world at a time when our economy is in crisis and Americans are already fighting two wars.”

McCain said it was even “more troubling” that Biden suggested supporters stick by Obama if the actions he takes are wrong or unpopular.

“Senator Obama won’t have the right response, and we know that because we’ve seen the wrong response from him over and over during this campaign,” he said.

“It has to mean that Joe Biden continues to harbor serious doubts about whether Barack Obama is prepared to be commander in chief,” Giuliani said.

The Obama-Biden camp downplayed the Delaware senator’s comments.

“With our nation facing two wars and 21st-century threats abroad, Senator Biden referenced the simple fact that history shows presidents face challenges from day one,” said campaign spokesman David Wade.

“After eight years of a failed foreign policy, we need Barack Obama‘s good judgment and steady leadership, not the erratic and ideological Bush-McCain approach,” he said.

Obama also announced last night he is canceling campaign events Thursday and Friday to visit his ailing grandmother in Hawaii.

Madelyn Dunham, 85, is at home in Honolulu after being hospitalized late last week, said campaign spokesman Robert Gibbs. He said her situation is “very serious,” but didn’t disclose her ailment.

Before the announcement, Obama, stumping with Hillary Clinton in Florida – where early voting began today with record crowds heading to the polls and residents waiting hours to cast ballots – invoked a new twist on a famous Ronald Reagan line to criticize GOP handling of the financial crisis.

“At this rate, the question isn’t just ‘Are you better off than you were four years ago? It’s ‘Are you better off than you were four weeks ago?’ ” he told a roaring crowd of 8,000 at George Steinbrenner Stadium in Tampa.

The two also sparred again on taxes.

McCain insisted he was the real tax cutter and hammered Obama again as a socialist-leaning liberal “who wants to redistribute the wealth” by hiking levies on people making more than $250,000 a year and giving breaks to those who pay no taxes.

.

Obama returned fire.

“John McCain calls that socialism. What he forgets, conveniently, is that just a few years ago, he himself said those Bush tax cuts were irresponsible. He said he couldn’t in good conscience support tax cuts where the benefits went to the wealthy at the expense of middle-class Americans,” he said.